1. Field of the Invention
The present invention relates to an illumination optical system for a microscope, and more specifically to an illumination optical system used in an observation in various magnifications.
2. Description of the Related Art
The magnification of a microscope used in an observation can be set in a wide range, and various functions may be demanded for an illumination optical system correspondingly. Generally, since an observation range is wider under lower magnification, the illumination optical system requires a function of illuminating a wider illumination range. On the other hand, an observation range is narrower under high magnification, but high resolution is needed. Therefore, the illumination optical system requires a high numerical aperture (NA). Thus, since the required function of the illumination optical system differs between an observation under high magnification and an observation under low magnification, the magnification of a microscope suitable for use is predetermined.
FIG. 1 is a conceptual drawing of an example of types of illumination optical system and a magnification appropriate for each type. As exemplified in FIG. 1, an illumination optical system can be designed and put to practical use as an illumination optical system of high NA type appropriate for an observation under high magnification, an illumination optical system of wide field-of-view type appropriate for an observation under low magnification, an illumination optical system of so-called swing-out type capable of switching between an illumination condition appropriate for high magnification and an illumination condition appropriate for low magnification by attaching/detaching a part of lenses of the illumination optical system, etc.
By exchanging between the above-mentioned illumination optical system of high NA type and illumination optical system of wide field-of-view type, or by a swing-out operation of attaching and detaching a part of lenses in the illumination optical system of swing-out type, an illumination environment can be optimized depending on the observation magnification. For example, the Japanese Laid-open Patent Publication No. H5-134190 and the Japanese Laid-open Patent Publication No. H8-101344 disclose the illumination optical system of swing-out type.
As described above, the conventional illumination optical system can optimize the illumination environment depending on the observation magnification. On the other hand, for example, when an observation is continued with the observation magnification changed from a low magnification (for example, lower than 40× magnification) to a high magnification (for example, 40× magnification or higher), the operation of the illumination optical system is required in addition to the operation of the observation optical system such as exchanging objectives etc. It is also required when the illumination optical system of swing-out type is used, thereby imposing a load on the user of the microscope.
For example, an immersion objective is used in an observation under high magnification exceeding 400× magnification (for example, 1000× magnification). However, after the immersion objective is used, oil etc. remaining on a sample is to be wiped off the sample. Since such an additional operation is required, the frequency in use of high magnification (range A) with the immersion objective is not so high. In addition, under very low magnification (for example, 10× magnification), it is often hard to specify an observation point by screening. Therefore, there is few opportunities to use the very low magnification (range B), and the frequency in use is low. Thus, the observation magnification of a microscope includes both the magnification that frequency in use is high and the magnification that frequency in use is low.